Increasingly, resources such as heavy crude oils, bitumen, tar sands, shale oils, and hydrocarbons derived from liquefying coal are being utilized as hydrocarbon sources due to decreasing availability of easily accessed light sweet crude oil reservoirs. These resources are disadvantaged relative to light sweet crude oils, containing significant amounts of heavy hydrocarbon fractions such as residue and asphaltenes, and often containing significant amounts of sulfur, nitrogen, metals, and/or naphthenic acids. The disadvantaged crudes typically require a considerable amount of upgrading, for example by cracking and by hydrotreating, in order to obtain more valuable hydrocarbon products. Upgrading by cracking, either thermal cracking, hydrocracking and/or catalytic cracking, is also effective to partially convert heavy hydrocarbon fractions such as atmospheric or vacuum residues derived from refining a crude oil or hydrocarbons derived from liquefying coal into lighter, more valuable hydrocarbons.
Numerous processes have been developed to crack and treat disadvantaged crude oils and heavy hydrocarbon fractions to recover lighter hydrocarbons and to reduce metals, sulfur, nitrogen, and acidity of the hydrocarbon-containing material. For example, a hydrocarbon-containing feedstock may be cracked and hydrotreated by passing the hydrocarbon-containing feedstock over a catalyst located in a fixed bed catalyst reactor in the presence of hydrogen at a temperature effective to crack heavy hydrocarbons in the feedstock and/or to reduce the sulfur content, nitrogen content, metals content, and/or the acidity of the feedstock. Another commonly used method to crack and/or hydrotreat a hydrocarbon-containing feedstock is to disperse a catalyst in the feedstock and pass the feedstock and catalyst together with hydrogen through a slurry-bed, or fluid-bed, reactor operated at a temperature effective to crack heavy hydrocarbons in the feedstock and/or to reduce the sulfur content, nitrogen content, metals content, and/or the acidity of the feedstock. Examples of such slurry-bed or fluid-bed reactors include ebullating-bed reactors, plug-flow reactors, and bubble-column reactors.
Coke formation, however, is a particular problem in processes for cracking a hydrocarbon-containing feedstock having a relatively large amount of heavy hydrocarbons such as residue and asphaltenes. Substantial amounts of coke are formed in the current processes for cracking heavy hydrocarbon-containing feedstocks, limiting the yield of lighter molecular weight hydrocarbons that can be recovered and decreasing the efficiency of the cracking process by limiting the extent of hydrocarbon conversion that can be effected per cracking step in the process, for example, by deactivating the catalysts used in the process.
Cracking heavy hydrocarbons involves breaking bonds of the hydrocarbons, particularly carbon-carbon bonds, thereby forming two hydrocarbon radicals for each carbon-carbon bond that is cracked in a hydrocarbon molecule. Numerous reaction paths are available to the cracked hydrocarbon radicals, the most important being: 1) reaction with a hydrogen donor to form a stable hydrocarbon molecule that is smaller in terms of molecular weight than the original hydrocarbon from which it was derived; and 2) reaction with another hydrocarbon or another hydrocarbon radical to form a hydrocarbon molecule larger in terms of molecular weight than the cracked hydrocarbon radical—a process called annealation. The first reaction is desired, it produces hydrocarbons of lower molecular weight than the heavy hydrocarbons contained in the feedstock—and preferably produces naphtha, distillate, or gas oil hydrocarbons. The second reaction is undesired and leads to the production of coke as the reactive hydrocarbon radical combines with another hydrocarbon or hydrocarbon radical. Furthermore, the second reaction is autocatalytic since the growing coke particles are more reactive with the cracked hydrocarbon radicals than the hydrocarbon feedstock. Hydrocarbon-containing feedstocks having a relatively high concentration of heavy hydrocarbon molecules therein are particularly susceptible to coking due to the presence of a large quantity of high molecular weight hydrocarbons in the feedstock with which cracked hydrocarbon radicals may combine to form proto-coke or coke. As a result, cracking processes of heavy hydrocarbon-containing feedstocks have been limited by coke formation induced by the cracking reaction itself.
Numerous catalysts have been developed for use in processes for cracking disadvantaged hydrocarbon feedstocks, however, such catalysts have not eliminated problems associated with coking, and catalyst activity may be significantly reduced over time by accumulation of coke on the catalyst. Catalysts used in fixed catalyst bed reactors typically contain a Group VIB and/or Group VIII metal supported on a carrier formed of alumina, silica, or alumina-silica. The carrier is generally selected to possess acidic properties that catalytically facilitate cracking by promoting the formation of radical carbo-cation hydrocarbon species from cracked hydrocarbons. Fixed bed cracking catalysts are also generally porous and highly adsorptive, where the pores and pore size distribution of the catalysts are determined by the carrier on which active metals are placed. The pores and pore size distribution of such catalysts markedly affect the activity, selectivity, and the cracking reaction rate. The active Group VIB and/or Group VIII metals of the catalyst facilitate hydrogenation of the cracked hydrocarbon radicals. Such catalysts are commonly sulfided to activate the catalyst, either before contacting the catalyst with a disadvantaged hydrocarbon feed or in situ with the disadvantaged hydrocarbon feed.
Processes that utilize fixed bed catalysts to crack a heavy hydrocarbon-containing material suffer significantly from catalyst aging due to coke deposition on the catalyst over time. As noted above, coke and proto-coke formation occurs in cracking a hydrocarbon-containing material, and is particularly problematic when the hydrocarbon-containing material is a heavy hydrocarbon-containing material, for example, containing at least 20 wt. % pitch, residue, or asphaltenes. The coke that is formed in the cracking process deposits on the catalyst progressively over time, plugging the catalyst pores and covering the surface of the catalyst. The coked catalyst loses its catalytic activity and, ultimately, must be replaced. Furthermore, the cracking process must be conducted at relatively low cracking temperatures to prevent rapid deactivation of the catalyst by annealation leading to coke deposition.
Slurry catalyst processes have been utilized to address the problem of catalyst aging by coke deposition in the course of cracking a hydrocarbon-containing feedstock. Slurry catalyst particles are selected to be dispersible in the hydrocarbon-containing feedstock or in vaporized hydrocarbon-containing feedstock so the slurry catalysts circulate with the hydrocarbon-containing feedstock in the course of cracking the feedstock. The feedstock and the catalyst move together through the cracking reactor and are separated upon exiting the cracking reactor. Coke formed during the cracking reaction is separated from the feedstock, and any coke deposited on the catalyst may be removed from the catalyst by regenerating the catalyst. The regenerated catalyst may then be recirculated with fresh hydrocarbon-containing feedstock through the cracking reactor. The process, therefore, is not affected by catalyst aging since fresh catalyst may be continually added into the cracking reactor, and catalyst upon which coke has been deposited may be continually regenerated.
Other slurry catalysts have been used in slurry cracking processes for the purpose of seeding the formation of coke. Very small particle slurry catalysts may be dispersed in a hydrocarbon-containing feedstock for the purpose of providing a plethora of small sites upon which coke may deposit in the course of the cracking process. This inhibits the formation of large coke particles since the coke may be dispersed throughout the hydrocarbon-containing feedstock on the small catalyst particles.
U.S. Pat. No. 4,557,821 provides a slurry catalyst formed of dispersed particles of highly active molybdenum disulfide useful for cracking a hydrocarbon-containing feedstock. The slurry catalyst exists as a substantially homogeneous dispersion of small particles in oil, where the catalyst's activity is dependent on the smallness of the particle size and resultant relatively large surface area rather than its pore characteristics. The catalyst does not have a porous support, e.g. a silica, alumina, or silica-alumina carrier, but is formed substantially only of molybdenum sulfides and molybdenum oxy-sulfides.
Although presently known slurry catalysts and slurry cracking processes utilizing such catalysts do not suffer the catalyst aging problems of fixed bed catalysts and fixed bed catalyst processes in cracking a heavy hydrocarbon-containing feedstock, coking is still a significant problem. Coking limits the yield of lighter molecular weight hydrocarbons that can be recovered from the cracking process since a portion of the hydrocarbons in the hydrocarbon-containing feedstock are converted to coke rather than to the desired lighter molecular weight hydrocarbons. Coking also decreases the efficiency of the cracking process by limiting the extent of hydrocarbon conversion that can be effected per cracking step in the process, even in a slurry process, since the hydrocarbon-containing feedstock and the catalyst must be periodically removed from the cracking process to separate developing coke particles to prevent excessive coking. The slurry catalysts may actually increase coking, for example, the slurry catalyst disclosed in U.S. Pat. No. 4,557,821 is described as “a very active coking catalyst”, and a process is disclosed therein for using such a slurry catalyst that requires the use of exacting, slow heating steps to avoid massive coking.
Improved processes for cracking heavy hydrocarbon-containing feedstocks are desirable, particularly those in which coke formation is significantly reduced.